Are you by chance seeing/feeling the crop, where the food is stored? The crop will get quite large, it is located on the right side of the chicken in the front. The crop will fill (swell) when they eat and then go down as the food is digested. Some chickens eat a little bit more than others and their crop can be quite pronounced.

This article shows the anatomy of a chicken and a good photo of what a crop looks like:

Have you observed whether or not it is empty in the morning? Normally a chicken will eat right before roosting at night and the crop will empty (pooping) overnight. The crop should be empty (flat) or almost empty by morning.

Offer them oyster shell free choice - put it in a small container or throw some in the tractor for them to scratch at.

My personal preference is also to offer poultry grit free choice, that can also be put in a small container or throw a little in the tractor. Some birds may find the proper grit while foraging or digging in the dirt, but I'm one of those people that says, well, why not offer, if they take some good, if not...well, that's good too. A bag of grit will last a very long time, the chickens will eat a piece or two whenever they need it. Same with oyster shell, it will last a long time as well, they will eat what they need.